An electrostatic precipitator generally comprises, within the housing which is traversed by a dust-laden gas, arrays of corona-discharge electrodes, frequently referred to only as corona electrodes or discharge electrodes, in appropriate frames, flanked by dust-collecting electrodes of sheet or strip construction upon which the dust particles collect when a high voltage direct current source applies an electrostatic potential across the corona and collecting electrodes.
In principle, the electrostatic precipitator, by reason of the corona discharge at the discharge electrodes, charges the particles of dust with a polarity corresponding to that of the corona electrodes, whereupon the dust particles are attracted to and deposit upon the collecting electrodes. From time to time the collecting electrodes may be rapped to dislodge the collected dust and cause it to deposit in a bin or hopper from which the dust can be removed.
Such rappers are generally provided on dry-operated electrostatic precipitators, wet-operated precipitators frequently utilizing a liquid to carry off the collected dust.
The operation of dry-process electrostatic precipitators for the cleaning of exhaust gases from steel-making converters must often be interrupted because the corona electrodes fail by scaling after a relatively short time even though the gas temperature is usually not higher than 150.degree. C. to 250.degree. C.
Investigations have shown that this result may be due to a smoldering of pyrophoric dust which has been deposited. It here may be noted that such pyrophoric dust, while tending primarily to deposit upon the collecting electrodes may also deposit to some extent to the corona electrodes should the dust have been oppositely charged.
Such smoldering on both electrodes may give rise to local temperatures in excess of 600.degree. C. so that, under the action of this smoldering, oxide layers on the electrodes which one normally would expect to have a protective effect, tend to spall off, especially when the electrodes are cleaned by rapping blows. Such rapping blows may be applied to the corona electrodes as well as the collecting electrode.
The spalling and scale formation is most noticeable on the corona electrodes which are held in frames, primarily because of the small ratio of cross section to surface area, is present but less in extent on the collecting electrodes, and is generally not observed on the tensioning frames which consist of tubes or the like having comparatively thick walls.
Since the corona electrodes are maintained under tension and are subjected to severe mechanical stresses, it is not possible to use corona electrodes which consist of nonscaling materials generally because these are incapable of resisting the mechanical stresses which are encountered in electrostatic precipitators and have a higher coefficient of thermal expansion than the conventional structural steels from which the tensioning frame may be made.
Consequently, under the conditions previously described, the expansion of the corona electrodes with heating may exceed that of the frames and as a result the corona electrodes could relax and could readily deform from their strict rectilinear tensioned conditions. Under the influence of the gas flow, oscillations might be generated in the bowed corona electrodes and the spacing between these electrodes and the collecting electrodes would fluctuate so that local regions of increased voltage gradient might be established to the detriment of efficient separation. Uncontrolled motion of corona electrodes in the context of an electrostatic precipitator generally cannot be tolerated.
Furthermore, relaxed or loose corona electrodes cannot have dust efficiently dislodged therefrom by rapping blows to the frame. Because of dust accumulations on the corona electrodes, efficiency of separation falls further.
The obvious solution to this problem is to construct both the tensioning frames and the corona electrodes spanning same of the same nonscaling materials. However, this is not practical in most cases for mechanical reasons. Tensioning frames of nonscaling materials not only are expensive because of the materials which are used, but also because of the difficulty in fabrication and assembly.